The invention concerns an electromagnetically actuated, single-surface friction coupling without a rotor slip-ring, having a pot-shaped armature element.
An electromagnetically actuated friction coupling with an armature element of this type is shown in the not previously published document DE 199 51 630. This coupling comprises a rotor element mounted so that it can rotate, a fixed magnet body with a magnet coil, and a rotatable pot-like armature element which, however, is mounted so that it can be displaced axially. The armature element has a ring disc shaped area and a cylindrical jacket shaped area. With its cylindrical jacket shaped area the armature element projects into an annular groove of the magnet body in such manner that magnetic flux can be transferred via a narrow, radial air gap. Electromagnetically actuated couplings of this structural form can be manufactured inexpensively, take up little structural space, and are very reliable. A coupling of this type is suitable in hand-operated percussion drilling machines, for the coupling or release, as desired, of the drive motor from the drilling tool on the drive output side. The coupling reacts to high loads and/or deflection movements of the housing of the percussion drilling machine, such as those that occur when the drilling tool gets stuck. In this way, risk of injury to the machine operator can be avoided. The coupling described in DE 199 51 630 is designed such that when fitted in a percussion drilling machine, the rotor element is in permanent drive connection with the drive motor and the armature element is in permanent drive connection with the drilling tool or the drilling tool holder.
It has been shown that owing to the gear ratios on the drive output side, when the coupling has been released and until the elements on the drive output side have come to a complete stop, undesired reaction torques can still act upon the housing of the percussion drilling machine.
A further problem is that when the coupling has been released, although the drive output side with the drilling tool comes to rest almost at once, the drive input side of the coupling and the drive motor still continue running for a few seconds even after the current supply to the drive motor has been cut off, because of the stored rotation impulse. The machine is not ready for use again until the drive motor has come to rest. Since as a rule it is desired to resume work with the drilling tool quickly after a coupling release process, this time taken for the drive input side to come to rest is perceived as unacceptably long.
The purpose of the present invention is therefore to reduce to a minimum the undesired reaction torques that can still occur after the coupling has been released. The coupling should also be developed further in such manner that the time taken for the drive input side to come to rest after a coupling release process, is made shorter.
These objectives are achieved by an electromagnetically actuated, single-surface friction coupling without a rotor slip-ring, having the characteristics of the principal claim. According to this, the rotor element is arranged on the drive output side and the armature element on the drive input side. By virtue of its pot-like shape, associated with its cylindrical jacket surface the armature element has a considerable fraction of its total mass a large distance away from the centre of rotation, and it therefore has a large moment of inertia. In contrast, the rotor element has a smaller diameter and a smaller proportion of its mass far removed from the centre of rotation. Its moment of inertia is therefore very much lower, so that the rotation impulse that has to be dissipated on the drive output side once the coupling has been released, is very much smaller. xe2x80x98Snatchingxe2x80x99 of a hand-operated tool after the coupling release process is accordingly reduced to a minimum.
In an advantageous embodiment of the invention, the friction surface of the rotor element is associated with the side of the ring disc area of the armature element opposite the cylindrical jacket shaped area of the armature element. In the engaged condition of the coupling the friction surface of the rotor element is in contact, so to speak, outside the pot-shaped armature on the bottom of the pot, such that the rotor element is located substantially outside the axial structural space occupied by the armature element. Accordingly, the rotor element is very close to the drive output side, which enables a short shaft with correspondingly low moment of inertia to be used on the drive output side.
Corresponding to the larger moment of inertia on the drive input side of the coupling, after a coupling release process a higher rotation impulse remains in the system on the drive input side. To shorten the time taken for the drive input side to come to rest after a coupling release process, in an advantageous embodiment of the invention the armature element can move axially between two end positions, in the first of which it is frictionally engaged with the rotor element while in the second end position it is in frictional contact with a fixed braking ring.
This embodiment can advantageously be developed further in that the armature element is or can be connected to an armature shaft by means of an axially deflectable spring element, such that the armature element is pre-stressed against the braking ring by the spring element. In the absence of current supply, as after a release of the coupling, this simply constructed brake is actuated by the pre-stressing force of the spring element. Particularly in combination with an embodiment in which the friction surface of the rotor element is associated with the side of the ring disc shaped area of the armature element opposite the cylindrical jacket shaped area of the armature element, a coupling-brake combination is provided by exceptionally simple means. The armature element then has a friction surface at each of its two axial ends. One friction surface is arranged on the ring disc shaped area of the armature, so to speak on the bottom of the pot opposite the rotor element, and one friction surface is arranged on the end of the cylindrical jacket shaped area opposite the braking ring. The friction surfaces of the rotor element and of the braking ring form axial abutments between which the armature element can move to and fro.
A design of the coupling which is favorable in terms of structural space is characterized in that a flange is provided on the armature shaft for connection to the spring element, which supports the armature element and which conducts a magnetic flux between the magnet body and the rotor element when current is flowing through the magnet coil. If a first part of the flange extends with a narrow radial air gap axially along an inner jacket surface of the magnet body and a second part of the flange surrounds a hub of the rotor element with a narrow radial air gap, the magnetic flux can be transferred with little loss, respectively across the narrow radial air gaps between the components rotating at different speeds. Besides the function of mechanically coupling the armature element to the armature shaft on the drive input side, the flange also fulfils the function of coupling the rotor element magnetically with the magnet body.
The electromagnetically actuated coupling according to the invention and its embodiments are suitable for coupling or releasing a drive motor and a drilling or percussion drilling tool in a hand-operated drilling or percussion drilling machine, as desired, thanks to their small moment of inertia on the drive output side, their compact structural form and their low manufacturing cost.